1. Field of the Invention
The present invention relates generally to image forming apparatuses such as copying machines, printers and facsimile machines that form images by using, for example, an electrophotographic scheme and an electrostatic recording scheme. More specifically, the invention relates to an intermediate-transfer-type image forming apparatus.
2. Description of the Related Art
In recent years, in the field of electrophotographic multicolor or full-color image forming apparatuses, so-called “in-line image forming apparatuses” are brought into practical use. The in-line image forming apparatuses have a plurality of photosensitive drums aligned in one line in units of a color, in which individual color toner images formed onto the individual photosensitive drums are sequentially superposed on an intermediate transfer member to form a color image.
FIG. 6 depicts an example of a conventional in-line electrophotographic full-color image forming apparatus (full-color copying machine) having an intermediate transfer member of the type described above. The image forming apparatus is configured of a printer section Pr and a reader section Sc. In this apparatus, image information of an original document sheet is obtained in the reader section Sc, and the image information is imaged over a transfer medium, such as paper, in the printer section Pr.
Image forming processes of the image forming apparatus will be outlined hereinbelow. When the image information is acquired from the original document sheet in the reader section Sc, as described above, light exposure corresponding to th image information is performed by an exposure apparatus 101. Then, electrostatic latent images are formed on photosensitive drums 103 disposed in an image forming section 102. The individual drums 103 are assigned for handling yellow, magenta, cyan, and black toner images. Toners are then supplied onto the latent images in the image forming section 102, and toner images are then formed. The toner images are primary-transferred onto an intermediate transfer belt 104, and are then secondary-transferred onto a transfer medium P in a secondary transfer section Te in a nip section defined between a secondary-transfer opponent roller 105 and a secondary transfer roller 106. Finally, the secondary-transferred toner images are fixed to the transfer medium P with pressure and heat in fixing means 107, and a permanent image is formed on the transfer medium P.
In the intermediate-transfer-type image forming apparatus as described above (refer to Japanese Patent Application Laid-Open No. 2001-066948, for example), since, in particular, the endless-belt-type intermediate transfer belt 104 is used as the intermediate transfer member, a degree of freedom takes place for disposing members such as a transfer-medium transport section 108, the secondary transfer section Te, and the fixing means 107. Reportedly, this enables shortening a transfer-medium transport path for the distance from the transfer-medium transport section 108 to the fixing means 107 via the secondary transfer section Te, therefore providing an advantage in miniaturization of the apparatus.
However, it has been pointed out that shortening the transfer-medium transport path creates the problem of causing mutual interference between a transfer process in the secondary transfer section Te and a fixing process in the fixing means 107.
In more detail, in the event that toner images are transferred to the transfer medium in the secondary transfer section Te and are fixed to the transfer medium P in the fixing means 107, a high quality image cannot be obtained unless the transfer medium P is transported at the speed controlled to be as constant as possible. However, since members, such as the secondary transfer section Te and the fixing means 107, are disposed close to each other, the fixing process in the fixing means 107 is undesirably commenced before completion of the transfer process in the secondary transfer section Te. At this event, suppose a case occurs in which, for example, the transfer-medium transport speed in the secondary transfer section Te is lower than the transfer-medium transport speed in the fixing means 107. In this case, the transfer medium P nipped in the secondary transfer section Te during the transfer process is forcedly pulled by the fixing means 107, thereby likely leading to an image defect.
Generally, to prevent the problem, the apparatus is designed such that a speed difference is provided between the transfer-medium transport speed in the secondary transfer section Te and the transfer-medium transport speed in the fixing means 107 (specifically, the transfer-medium transport speed in the fixing means 107 is set relatively lower). This causes the transfer medium P to form curves between the secondary transfer section Te and the fixing means 107 so that the curves sever as buffers to prevent the processes of the secondary transfer section Te and the fixing means 107 from being interfered with each other.
The transfer-medium transport speeds in the secondary transfer section Te and the fixing means 107 are influenced by factors, such as the type of the transfer medium, the density of the formed image, the operation environment, and the component durability. As such, it is very difficult to accurately control the speeds. For this reason, the speed difference between the transfer-medium transport speeds in the secondary transfer section Te and the fixing means 107 should be set with sufficient margins by taking all applicable conditions into account.
Under these circumstances, as shown in FIG. 7, depending on the condition, the curve between the secondary transfer section Te and the fixing means 107 can be grown larger. In this state, the image on the transfer medium P between the secondary transfer section Te and the fixing means 107 is unfixed. As such, when the curve is grown larger to an extent of bringing the transfer medium into contact with an interior portion of the apparatus main body, an image defect may occur, possibly entailing contamination of the apparatus interior. In addition, to prevent such contact with an interior portion of the apparatus, a sufficiently large spacing should be provided, thereby making it disadvantageous for miniaturization of the apparatus.
Further, as the curve is grown larger, the degree of freedom of the transfer medium P between the secondary transfer section Te and the fixing means 107 is increased. As such, wobbling, waviness, and/or the like of the transfer medium P can influence the secondary transfer section Te, the fixing means 107, and the like, thereby leading to the problem of causing, for example, an image defect and/or wrinkle of the transfer medium P.